Color removal from acrylonitrile polymers



Patented Dec. 9, 1947 COLOR REMOVAL FROM ACRYLONITRILE POLYMERS Robert Albert Scheiderbauer, Kenmore, N. Y., assignor to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware No Drawing. Application January 10, 1945,

Serial No. 572,260

2 Claims. 1

This invention relates to the removal of. color from shaped articles comprising a polymer of acrylonitrile and is particularly concerned with the bleaching of yarn composed of acrylonitrile polymer.

Polymers of acrylonitrile such as acrylonitrile polymers in which the polymers are composed entirely of acrylonitrile, and interpolymers and copolymers of acrylonitrile with other materials such as vinyl chloride, vinyl acetate, acrylic acid, its esters and homologues, styrene, butadiene, isobutylene and other polymerizable substances, especially those containing olefinic and diolefinic linkages, although potentially of great value on account of their highly desirable properties, are insoluble in ordinary organic solvents when the polymers contain at least 85% by weight of acrylonitrile (vinyl cyanide) in the polymer molecule. Solutions of such polymers in organic solvents such as dimethyl formamide and other organic solvents, as well as their conversion by wet and dry or evaporative processes into various articles such as filaments, films, etc., are described and claimed in the copending applications of George Henry Latham, Serial No. 447,466, filed June 17, 1942, and Ray Clyde Houtz, Serial No. 447,446, filed June 17, 1942. This invention is broadly concerned with polymers of acrylonitrile containing at least 85% by weight of acrylonitrile in the polymer molecule although it is most particularly concerned with and will be specifically described in connection with polyacrylonitrile, i. e. the polymer which is composed substantially entirely of acrylonitrile in the polymer molecule.

One method which may be used to produce Dolyacrylonitrile yarn is the so-called dry" or evaporative spinning process which comprises extruding a solution of polyacrylonitrile through a nozzle into air or other gas inert to the polymer, maintained at such a temperature and under such other conditions as will cause the complete vaporization of the solvent contained in the polvymer solution. The yarn may be subjected, if desired, to a stretching operation in order to increase tenacity as well as otherwise to improve physical properties by orienting the micelles of which the yarn is composed. The stretching may be efiected by subjecting the yarn at an elevated temperature of the order of 100 and above, to a tension adequate to effect the desired degree of stretching, this stretching procedure being described and claimed in the copending application of Daniel T. Meloon Serial No. 496,397, filed July 28, 1943.

60 The following examples in which parts, perthe polymer solution and coagulate or precipitate the polymer from its solution, the yarn being carried through the bath for a period of time sufiicient to solidify the polymer to the desired extent and being subjected, if desired, to a stretching operation, preferably while it is in the gel state, in order to increase tenacity as well as otherwise to improve physical properties by orienting the micelles of which the filaments are composed, the yarn being washed, if desired, to remove the coagulating bath liquid and dried.

Yarn produced by the dry and wet spinning processes described above has excellent physical properties, but does not possess that degree of freedom from color which is required by the exacting standards of the textile industry. Moreover, the customary methods utilized inthe textile art for the bleaching of color from filaments, fibers and the like do not effect the desired elimination of color from polyacrylonitrile yarn. The yellowish or golden-brown color present in polyacrylonitrile articles occurs either during polymerization or processing of the polymer prior to or during shaping of the polymer to the desired form and may be caused by prolonged treatment of the polymer at high temperature, e. g. 100 C. and above.

The invention will be described most particularly with reference to yarn composed of continuous filaments, but it will be understood that this is merely by way of illustration since the invention may also be applied to staple fibers, thin transparent films such as may be used as Wrapping tissue, monofils, artificial straw, hollow pellicles such as tubing and other industrial products composed of acrylonitrile polymer.

It is an object of this invention to provide a process for removing color from articles composed of acrylonitrile polymer. It is a particular object to produce color-free acrylonitrile polymer filaments, fibers and yarns, without substantially impairing the physical properties of the products. Other objects will appear hereinafter.

The objects of the invention are accomplished in general by spinning polyacrylonitrile yarn by a dry or by a wet spinning process, preferably collecting the dry yarn on a bobbin or other suitable collecting device and soaking the dry yarn for a short period of time in an aqueous solution of hypochlorous acid at a temperature of at least 90 C., the solution when temperatures above the boiling point at atmospheric pressure are used, i. e. temperatures of 100 C. or above, being maintained in the liquid state by the use of superat- 55 mospheric pressure, e. g. in an autoclave. Ob-

jectionable coloring present in the yarn is removed by the bleaching treatment with no substantial impairment of the desired physical properties of the yarn.

centages and proportions are by weight unless otherwise specified and which are intended to be illustrative instead of limitative, set forth preferred forms of the invention.

Example A Yarn to be treated in accordance with the present invention may be prepared as follows:

A solution of 100 parts of polyacrylonitrile possessing an average molecular weight of 60,000 as determined from viscosity measurements by'the Staudinger formula, in 300 parts of dimethylformamide, is extruded at the rate of 10.1 grams per minute and at a temperature of 130 C. downwardly through a spinneret having ten holes 0.125 mm. in diameter, into a tubular cell through which heated air is circulated in an upward direction. The walls of the cell are maintained at a temperature of approximately 400 C. The fresh air enters the lower portion of the chamber at a temperature of about 100 C. while the waste air leaving the upper portion of the cell is at a temperature of about 200 C. The cell has a length of 8 feet, sufiicient for the evaporation of the dimethyl formamide from the extruded solution. The solidified, multifilament polyacrylonitrile yarn issuing from the bottom of the cell is collected on a rotating bobbin at a speed of 100 yards per minute. The yarn, which has a denier of 248, is soft and pliable and has a dry tenacity of 0.62 gram per denier and a wet tenacity of 0.53 gram per denier. Its elementary analysis corresponds to that of the polymer of acrylonitrile originally dissolved in the dimethyl formamide. The collected yarn is then unwound from the collecting bobbin, passed without slippage about a positively driven roller, heated to a temperature of 140 C. and wound upon a rotating bobbin, the peripheral speed of which is eight times that of the heated roller. The resulting yarn which has been permanently stretched by this treatment has a dry tenacity of 3.4 grams per denier.

Example I A solution of sodium hypochlorite containing 025% available chlorine was acidified with sulfuric acid to a pH of 2.1. Three hundred (300) cc. of this solution were placed in an autoclave and a skein containing 45 yards of polyacrylonitrile yarn prepared by. a dry spinning process such as that described in Example A was immersed in the bath. The autoclave was closed and steam was introduced, the autoclave being bled to remove air. The steam pressure. was allowed ultimately to build up to 25 pounds, at which pressure it was maintained for one half hour (at a temperature of approximately 130 C.) At the end of that time, the autoclave was bled off, the skein was removed, washed with cold distilled water and dried. Physical properties were measured as follows: Before treatment, the yarn showed a dry tenacity of 3.3 grams per denier and a dry elongation of 14.7%, but after treatment, the dry tenacity was 3.3 grams per denier and the dry elongation 16%. The bleach rating was 1 on an arbitrary scale in which zero would represent perfect bleaching, as compared with a color rating of 4 for the same yarn which was not subjected to the bleaching treatment.

By comparison, gel polyacrylonitrile yarn which was wet spun by extrusion of the solution through a nozzle into a coagulating liquid was collected, subjected to the same bleaching treatment, but with only 0.125% available chlorine present and after being dried, showed a decrease in dry te- 4 nacity of about 28%, compared with the dried, wet-spun yarn which had not been subjected to the bleaching treatment, and showed a color rating of 3.

Example II A 50-yard skein of polyacrylonitrile yarn prepared by a dry-spinning process such as that described in Example ,A was placed in 300 ml. of a. bleaching solution made from sodium hypochlorlte (0.25% available chlorine) and acidified with sulfuric acid to a pH of 2.0. The solution contained in a 400 ml. beaker covered with a watch glass was placed in a steam bath and was heated to 97 C. Bleaching action first became pronounced when the temperature rose to about C. After about a thirty minute treatment, no further bleaching took place. The yarn was then washed and dried as in Example I. The degree of freedom from color was at least as good as in Example I. The bleached yarn had a dry tenacity of 4.24 grams per denier and a. dry elongation of 14.8% as compared with a. dry tenacity of 4.21 grams per denier and a dry elongation of 14.5% for the same yarn before the bleaching treatment, this comparison showing that the bleaching treatment did not impair the physical properties of the yarn.

While the invention is particularly applicable to the bleaching of dry-spun polyacrylonitrile yarn, it may also be applied with corresponding benefit to the bleaching of wet-spun polyacrylonitrile yarn which has been dried as shown in the following example.

Example III Dried polyacrylonitrile yarn obtained by a wetspinning process, exhibiting a dry tenacity of 4.49 grams per denier and a dry elongation of 16.9% and having a color rating of 10 was treated in a covered beaker at 97 C. and at atmospheric pressure as was the yarn of Example II with the same bleaching solution of Example II having 0.25% available chlorine and a pH of 2.0, followed by washing with cold distilled water and drying, the periods of bleaching being respectively five minutes and thirty minutes. Two other experiments were run under the same conditions except that the bleachin solution contained 0.125% available chlorine with a pH of about 2.0 for respective periods of five and thirty minutes. The results of these experiments are tabulated as follows:

Concentration of Bleach Per cent Available Chlorine Dry Elongstion Time of pH of Bleaching Dry Te- Solution nacity Color Rating NPN COCO rewcuoo awe Example IV The acrylonitrile polymer for use with this inuseful effect, in the customary concentrations and vention is preferably prepared by the ammonium at temperatures below those used in the practice persulfate catalyzed polymerization of monoof the present invention, as far as acrylonitrile meric acrylonitrile dissolved or emulsified in wapolymer yarn is concerned. Alternatively, the ter. It can, however, be prepared by any other 5 use of the relatively concentrated solutions of suitable type of polymerization reaction, such as bleaching agent used in accordance with this infor example the emulsion type reaction disclosed vention would so degrade the said textile mateby U. 8. Patent No. 2,160,054 to Bauer et al. The rials as to make them practically useless. polymer preferably possesses a molecular weight The bleaching treatment of this invention may within the range 15,000 to 250,000 or even higher, 10 be applied to the yarn after the removal of the as calculated from viscosity measurements by the solvent (when the yarn is dry-spun), or after Staudinger equation: I drying (when the yarn is wet-spun) and before I N collection of the yarn although the treatment is Molecular w g 'a, preferably applied after the dry yarn is wound wherein. on a bobbin which should be perforated to allow easy permeation of the yarn, or when the yarn K.=1 5 -4 is in the form of skeins.

By the process of the present invention, poly- N ifi viscosity of solution 1 acrylonitrlle yarn, filaments, fibers, film, mono- 6 viscosity of l nt filaments, artificial straw, hollow tubing and other structures can be bleached to a satisfactory colorless or white character without deleterious ac- C'=concentration of the solution expressed as the on the structure- The bleached p y l'y number of moles of the monomer (calculat d) onitrile structures of the present invention, ex-

per liter of solution. 25 cent for improved color, exhibit substantially the The molecular weight of the polymer obtained is 3: prysmal characteristics as the unbleached s c u es.

de dent n uch facto h on r 11 z z m the i: 523: 523: Since it is obvious that many changes and moditype catalyst present, the temperature of the flcations can be made in the above described dereaction, etc. When the monomer is present in if ig z g g f g ig 5 e0 1 o e nven on. s o e un ers 00 a maintained at a tempera the invention is not to be limited to the details ture of from 3 to 5 C., it is found that the use of 4% of ammonium persulfate catalyst (based on described herein except as set forth m the up pended claims.

the :veight of the acrylonitrile) results in the foras I d mat on of a polymer having a molecular weight (as calculated by the above equation) of approxi- In the process of removing color from an match, 60,000 Increasing or decreasing the article composed of a polymer of acrylonitrile amount of the catalyst, while maintaining the containing at least 85% acrylonitme in the polyother conditions constant, decreases or increases met molecule the step which comprises subjectthe molecular weight of the Dolmen o ing said article after it has been dried to treat- The aqueous, hypocmomus acid solutions used ment with an aqueous solution containing hypoin accordance with the present invention will morons acid corresponding to o'o5% to 19% by contain 095% to 19% available chlorine and weight of said solution of available chlorine and preferably 0.1% to 0.5%,with the most preferred at temperature 90' to 150 and for a concentration being in the neighborhood of period not exceeding thirty minutes 025% hypochlorous acid solution may readily be In the Pmess remm'mg mm yam prepared by dissolving a material such as sodium mmposed of polymer of acrymmmle containing hypoehlorite or calcium hypochlorlte, or other at east 85% acrylllorlilitrile ini the pgllynfir molciea soluble hlorite, in ater, idlf cule, the step w c compr ses su ec ng sa hypoc w ac mg with been dried to treatment with mineral acid such as hydrochloric or sulfuric acid mm after it has to a pH of less than 7.0 and preferably 2.0 to 5.0, an 5011mm cmwnmg acid the term "available chlorine i if i the corresponding to 0.05% to 1.0% by weight of said chlorine present as hypochlomus Mm An excess solution of available chlorine at a temperature of of mineral acid added to the hypochlorite solution to and for period not exceeding does no harm and the pH can be lower than 2.0 thirty minutes or even close to 0.0, if desired. mm) m The temperature for the bleaching treatment ROBERT ALBERT ERBA should be within the range to 150 C., and REFERENCES mm preferably 90' to C., with the pressure being sumciently high to maintain the treating solution 60 The following references are of record in the liquid. The time of treatment will be short and file of this Patent! preferably thirty minutes or less, but will depend on the temperature and the available chlo- UmED STATES PATENTS rine concentration. It is preferred that the ves- Number Name Date 8 in which the bleaching treatment i carri d 65 2,021,763 Bauer Nov. 19, 1935 on be covered; for example, as in the case of an 2,226,353 McCarthy Dec. 24.

autoclave where the treatment i carried out 2,292,737 Biomer Aug. 11, 19 2 der pressure as in Example I, or by the use of a 2.295.660 Hubert/ills: p 15, 1942 watch glass or its equivalent as is described in Example 11 where the treatment is not carried out to FOREIGN PATENTS under pressure. Number Country Date Sodium hypochlorite has been used in very di- 437,271 Great Britain Oct. 28, 1935 lute concentrations for the treatment of textile 518,970 Great Britain Mar. 13, 1940 fibers such as cotton and rayen, but it has no 302,840 France Sept. 16, 1936 

